Thank you for reaching out. The reason for the slowness is that the column(s) in question are not indexed in the des_dr1.mag table. For the example you provided, you can get the same information from the main table much faster:
print(qc.query('select mag_auto_g from des_dr1.main where coadd_object_id = 171179340'))
CPU times: user 26 ms, sys: 0 ns, total: 26 ms
Wall time: 114 ms
Let us know if this will do. We can also compute indices on the mag table, in case that only that table has columns that you need.
Robert for the DL team